The invention relates to a power sensor for measuring the average power of modulated or unmodulated radio-frequency or microwave signals over a large dynamic range.
A very wide variety of embodiments of power sensors are known. The most common versions contain either a single thermal sensor, which produces an electrical measurement quantity proportional to the electrical power absorbed, or contain a single diode rectifier in a one- or two-way circuit, which delivers an electrical output quantity equivalent to the voltage across the terminating resistor, from which the power to be measured can then be determined. Whereas power sensors with diodes can measure accurately the power of CW signals (unmodulated sine-wave signals) over a range of from xe2x88x9270 dBm to +20 dBm, their use in relation to envelope-modulated signals is restricted to the diode""s so-called quadratic characteristic-response region from xe2x88x9270 dBm to approximately xe2x88x9220 dBm. The dynamic range of about 50 dBm which can be achieved here is significantly smaller than in the case of CW signals, and is roughly the same size as in the case of thermal sensors.
In particular, the requirements of the communication standard for the second and third generations of digital mobile telephony have led to the development of a new group of power sensors with a substantially larger dynamic range for modulated signals. In one of these known power sensors, two sensor branches are provided and, specifically, a first sensor branch for measuring in a low power measurement range is provided directly at an input, and a second sensor branch for measuring in a high power measurement range is connected to the input via a special resistor network which serves both as a characteristic impedance-matched termination for the first sensor branch and also to provide the power feed for the second sensor branch (U.S. Pat. No. 4,943,764). In the practical embodiment of this known power sensor, the sensitive input-side sensor branch needs to be turned off when the other sensor branch for the high powers is being used (Hogan, R.: Wide-Range Sensor Gauges Power Of Complex Signals. Microwaves and RF, September 1999, pp. 128-137). The measurement range of the power sensor which is produced is specified as from xe2x88x9260 dBm to +20 dBm, which means that the sensitivity is 10 dB less than is achievable with known power sensors for CW signals (datasheet xe2x80x9cEPM Series Power Meters, E-Series and 8480 Series Power Sensorsxe2x80x9d, literature number 5965-6382E from Agilent Technologies). The sensor is available in a 6 GHz version and in an 18 GHz version.
A power sensor with two sensor branches is also already known, which feeds the signal power to be measured via a power splitter into two sensor branches, respectively with different sizes of attenuators, in order to measure the signal power in a lower power range and in a high power range (U.S. Pat. No. 4,873,484). The power splitter used in this case is designed as a so-called three-resistor power splitter (Russel A. Johnson: Understanding Microwave Power Splitters, Microwave Journal Vol. 18, December 1975, pp. 49-56). In such sensors which operate with power splitters, it is also already known to arrange two such three-resistor power splitters (also referred to as resistive power dividers) in cascade, and hence to provide a total of three sensor branches for different power measurement ranges (Anritsu Co.: A Universal Power Sensor. Microwave Journal, March 2000, pp. 130-134). The measurement range specified by the manufacturer for this power sensor is likewise only from Jxe2x88x9260 dBm to +20 dBm. The sensor is available only in a 6 GHz version.
Lastly, in the case of power sensors with diodes, it is also already known to use a plurality of diodes connected in series in the same direction as a rectifying element, either in order to reduce the effect of the junction capacitance, which depends on the drive level, on the linearity of the sensor in the case of sensors which are used exclusively for CW signals (U.S. Pat. No. 5,204,613) or, in the case of sensors for modulated signals, in order increase the measurement range of a sensor branch (Hogan, R.: Wide-Range Sensor Gauges Power Of Complex Signals. Microwaves and RF, September 1999, pp. 128-137).
There is a need to provide a power sensor for measuring the power average value of modulated signals in the frequency range up to 18 GHz or higher, whose sensitivity and dynamic range are greater than in the case of the known solutions and are comparable with the properties of power sensors for CW signals.
This and other needs are addressed by the invention, in which at least three mutually independent sensor branches with correspondingly different power measurement ranges are provided, in order to divide up the required dynamic range of 90 dB so finely that the perturbing effects due to noise or errors, which occur at the measurement-range limits of the individual sensor branches during the RMS value rectification, can be kept sufficiently small. The individual sensor branches preferably contain diode detectors which, in a manner that is known per se, are constructed using a single rectifier diode (one-way rectifier) or two rectifier diodes with different polarities (two-way rectifier) and an associated charging capacitor. In order to achieve the high sensitivity, as in the case of a power sensor for CW signals, a first sensor branch is arranged directly at the input, whereas the other sensor branches are fed with correspondingly divided powers via power splitters and attenuators. The synchronization of the sensor branches as a function of frequency is particularly important in this case, since only with minor frequency-response differences is unproblematic changeover from the measurement results of one branch to those of another branch possible. This is not guaranteed in the case of the known solutions.
According to the invention, the synchronization problem is solved by the fact that the measurement quantity, that is, the wave impinging on the sensor, is sent with the least possible perturbation through the first sensor branch and subsequently is divided between the two other sensor branches by means of a power splitter, with substantially load-independent synchronous response (tracking), and the measurement device in the first sensor branch is to be configured in such a way that its measurement value is representative of the level of the power of the incident wave, irrespective of the matching of the power splitter. To that end, in the first sensor branch, a plurality of voltage taps, each with an allocated detector, are provided at suitable intervals on the feed line to the power splitter, and the sum of the output voltages of the detectors, or of the apparent powers which can be determined therefrom, is formed in a suitable way. The summation reduces the positional dependency of the measurement results, which is due to standing waves on the feed line, so that the power of the incident wave can be measured very accurately because it is being measured substantially independently of the matching of the power splitter and hence frequency-independently (Sucher, M.: Final Report on High Power Measuring Techniques; Microwave Research Institute, Report R-718-59, PIB-646, March 1959). A further advantage of such an arrangement with distributed measurement points is that the perturbations which are generated by the individual detectors partially cancel out one another and hence improve the input-side matching of the power sensor. Although the advantages of the described measurement arrangement are restricted to a frequency band of the order of one to two octaves, it is also expedient to use it in a broadband power sensor with a frequency band extending over several octaves, because perturbations due to mismatching of individual modules do not usually become relevant until the upper two thirds of the specified frequency range.
For symmetrical division of the measurement signal, independently of mismatches at the outputs of the power splitter, there are two viable embodiments of power splitters. The first group comprises those in which a reflected wave, due to mismatching of one output, is reflected back to the same degree as it is transmitted to the other output, expressed by the relationship s22=s32 and s33=s23 between the s parameters (gate 1: input of the power splitter). The group of this type of power splitters covers an arrangement whose low-frequency equivalent circuit diagram can be represented by one ohmic impedance each between the input and the two outputs, the resistance being equal to the characteristic impedance which is used (resistive power splitter). This arrangement is represented in FIG. 1 and FIG. 2.
Also applicable according to the invention, however, are power splitters which are matched on the output side and have a high degree of isolation between the outputs (s22≈0, s33≈0, |s23| less than  less than |s21|s31| and |s32| less than  less than |s31/s21|) because, in this case, a reflected wave due to connecting up one output is neither reflected back into this output nor transmitted to the other output, so that the ratio of the extracted powers remains constant. The group of this type of power splitters covers, inter alia, the so-called Wilkinson Divider (Webb, R. C.: Power Divider/Combiners: Small Size, Big Specs; Microwaves, November 1981, pp. 67-74).
Not applicable according to the invention are resistive power splitters whose low-frequency equivalent circuit diagram exhibits three equally large resistors which radiate from a star point and whose resistance is equal to one-third of the characteristic impedance. The fact that, with this type, a reflected wave due to connecting up one output is not reflected back into this output, but it is transmitted to the other output (s22≈0, s33≈0, |s23|≈0.25 and |s32|≈0.25) leads directly to an asymmetry in the power division. Since mismatching is in turn frequency-dependent, exact synchronization of the two output powers cannot be possible.
From the two outputs of the first power splitter it is possible to feed, via suitably dimensioned attenuators, two sensor branches whose power ranges are correspondingly staggered. It is also possible, however, to provide a further power splitter between the two outputs in the version according to the invention, so as to have three outputs available for a total of four sensor branches, which permits even finer division of the power range (FIG. 2).
In a power sensor according to the invention, it is advantageous not to use individual diodes within a detector, but rather a plurality of diodes connected in series, because it is thereby possible to increase the dynamic range of an individual sensor branch for the measurement of modulated signals. Associated with this is a gain in the spare driving capacity at the upper measurement limit of the power sensor, at the cost of only a comparatively minor loss of sensitivity at the lower measurement limit. Based on a power range of from xe2x88x9270 dBm to +20 dBm for a CW sensor with single diodes, the series connection of two diodes in each case raises the lower measurement limit of the sensor merely by 3 dB (halved sensitivity), whereas the spare driving capacity increases by 6 dB (halved input voltage per diode). With the series connection of 3 diodes, a shift of lower measurement limit by 5 dB and of the upper measurement limit by 10 dB is to be expected, and not until the series connection of 10 diodes is the xe2x88x9260 dBm measurement limit of the known solutions with a plurality of sensor branches reached.
Instead of series-connected diodes, it is also possible to use a circuit according to the commonly assigned German Patent Application 199 13 338, published on Sep. 28, 2000 and having an English-language title of xe2x80x9cMulti-path HF diode rectifier circuit with at least one rectifier diode and at least one charging capacitor, uses output from rectifier circuit that is loaded by same relative temperature coefficient resistance of rectifier diode,xe2x80x9d in order to increase the dynamic range of an individual sensor branch.
It is furthermore advantageous to evaluate the output signals of all the sensor branches at the same time, and to obtain the measurement result in the overlap ranges of two neighboring sensor branches from the output signals of both branches. The simultaneous acquisition and parallel further processing of the output voltages obtained from the rectifiers of the sensor branches is preferably carried out with an arrangement according to the commonly assigned German Patent Application 199 55 342.4 published on May 23, 2001, i.e. the output voltages of the diode rectifiers are converted into a digital value, and each of these digital values is weighted with weighting factors whose ratio is derived from the drive level of at least one of the diode rectifiers. After scaling to a common quantization unit, the digital values weighted in this way arc summed to form the actual digital measurement value and then evaluated. In the overlap ranges between two sensor branches, the measurement result is obtained from the output signals of both rectifiers, so that the effects due to noise or errors during the RMS value rectification at the measurement-range limits are averaged out. The measurement inaccuracy is therefore very small overall, and high reproducibility is guaranteed even in the overlap range, because the hysteresis necessary when switching over is not utilized. A high measurement rate is also achieved since a change of range is superfluous.
Still other aspects, features, and advantages of the present invention are readily apparent from the following detailed description, simply by illustrating a number of particular embodiments and implementations, including the best mode contemplated for carrying out the present invention. The present invention is also capable of other and different embodiments, and its several details can be modified in various obvious respects, all without departing from the spirit and scope of the present invention. Accordingly, the drawing and description are to be regarded as illustrative in nature, and not as restrictive.